FILE - In this Friday, Dec. 14, 2012, file photo provided by the Newtown Bee, a police officer leads two women and a child from Sandy Hook Elementary School in Newtown, Conn., where a gunman opened fire, killing 26 people, including 20 children. A prosecutor is planning to release a report on the investigation into the massacre at Sandy Hook Elementary School on Monday, Nov. 25, 2013, but the public will have to wait longer for the Connecticut state polices full accounting of the crime.(AP Photo/Newtown Bee, Shannon Hicks, File) MANDATORY CREDIT: NEWTOWN BEE, SHANNON HICKS

HARTFORD (AP) — Why Adam Lanza went on his murderous shooting rampage at a Newtown elementary school is a mystery and may never be known, prosecutors said in a report Monday as they closed their yearlong investigation.

Lanza, 20, was obsessed with mass murders and the 1999 Columbine High School shooting in particular, but wasn’t aggressive or threatening before the attack, according to the summary by the lead investigator, State’s Attorney Stephen Sedensky III.

The report describes a gunman who had “significant mental health issues” but had sure knowledge of what he was planning: He had materials on mass murder, he smashed his computer hard drive, and he used earplugs during the shooting.

Lanza killed 20 first-graders and six educators with a semi-automatic rifle inside Sandy Hook Elementary School on Dec. 14. He also shot his mother to death inside their home before driving to the school, and committed suicide with a handgun as police arrived.

“The obvious question that remains is: ‘Why did the shooter murder 27 people, including 20 children?’ Unfortunately, that question may never be answered conclusively,” the report said.

Lanza “was under no extreme emotional disturbance for which there was a reasonable explanation or excuse.”

Sedensky also said there was no clear indication why Lanza chose Sandy Hook Elementary other than that it was close to his home.

The report said noted that in 2005, he was diagnosed with Asperger’s disorder and lacked empathy for others. But “what contribution this made to the shootings, if any, is unknown.”

Asperger’s is an autism-like disorder that is not associated with violence.

Lanza might have hinted at his intentions online in the days before the massacre, the report indicated.

A Texas woman contacted Hartford police the day of the Dec. 14 attack to say her son had interacted with someone while playing a videogame 20 hours earlier who said there would be a school shooting, according to the report. It wasn’t clear from the report whether she contacted authorities before or after the massacre.

Also, two days before the shooting, an anonymous user posted comments online about planning to commit suicide Dec. 14 and saying it would make national news. The poster claimed to live in Connecticut.

But Sedensky said the hard drive taken from Lanza’s home was so damaged that data will probably never be extracted from it.

A timeline released with the report indicates that nearly six minutes passed between the arrival of the first Newtown police office and the time officers entered the school. The report said officers were operating under the belief there may have been more than one shooter.

Whether the delay made any difference was unclear. The report said Lanza killed himself about a minute after the first officer arrived.

“Some recalled that the shooter had been bullied; but others - including many teachers - saw nothing of the sort.”

Lanza’s mother, Nancy Lanza, was concerned for her son and said that he hadn’t gone anywhere in three months and would communicate with her by e-mail only, even though they were living in the same house. The mother never expressed fear that she or anyone was in danger from her son, the report said.

Donna Soto, the mother of slain teacher Victoria Soto, said in a statement that nothing could make sense of the shooting.

“Yes, we have read the report, no, we cannot make sense of why it happened. We don’t know if anyone ever will,” Soto wrote. “We don’t know if we will ever be whole again, we don’t know if we will go a day without pain, we don’t know if anything will ever make sense again.”

To try to figure out the motive, investigators said, they interviewed members of Lanza’s family — his father and brother cooperated fully — along with teachers and others. They said they also tried within the limits of privacy laws to gather information on his medical treatment.

They found no evidence the young man had taken any medication that would have affected his behavior or explain the bloodbath.

Sednesky’s report is a summary of a much larger Connecticut state police evidence file that is expected to be released at a later date.

Sedensky has gone to court to fight release of the 911 tapes from the school and resisted calls from Connecticut’s governor to divulge more information sooner.

The withholding of 911 recordings, which are routinely released in other cases, has been the subject of a legal battle between The Associated Press and Sedensky before the state’s Freedom of Information Commission, which ruled in favor of the AP, and now Connecticut’s court system.

A Connecticut judge said Monday he will listen to the 911 recordings from the school before ruling on whether they can be publicly released.

If the recordings are released, the AP would review the content and determine what, if any, of it would meet the news cooperative’s standards for publication.

The report said the first officer arrived behind the school at 9:39 a.m. Two other Newtown officers then arrived at the school within seconds, and gunshots were heard in the background.

The last gunshot officers heard, which is believed to be the suicide shot by Lanza, was heard at three seconds past 9:40. Newtown officers entered the school at 47 seconds past 9:44, according to the report.